Hydroxycinnamic acid-derived crosslinking dimers

ABSTRACT

A composition and article of manufacture comprising a crosslinked polymer and a method of crosslinking a polymer are disclosed. The crosslinked polymer includes a polysiloxane and at least two dimerizable hydroxycinnamic acid crosslinkers. The method includes obtaining a polysiloxane and at least two dimerizable hydroxycinnamic acid crosslinkers, reacting the polysiloxane and the crosslinkers to form a crosslinked polysiloxane, and dimerizing the at least two hydroxycinnamic acid crosslinkers.

BACKGROUND

The present disclosure relates to crosslinkers derived fromhydroxycinnamic acids and, more specifically, to hydroxycinnamicacid-derived crosslinkers for polysiloxanes.

Polydimethylsiloxane (PDMS) is a polysiloxane polymer with the formulaCH₃[Si(CH₃)₂O]_(n)Si(CH₃)₃, where n is an integer greater than 1. PDMSis used in a variety of applications. Examples of these applications caninclude medical devices, contact lenses, fabrication and prototyping ofmicrofluidic devices, soft lithography processes, shampoos, caulking,anti-foaming agent, foods and cooking oils, etc. PDMS can have varyingproperties depending on factors such as the length of the monomer chainand the density of crosslinking throughout the polymer matrix. The typeof crosslinking is determined, at least in part, by the molecules usedas crosslinkers. By varying the functional groups and structures oforganic crosslinkers, PDMS materials having numerous properties can beproduced.

SUMMARY

Various embodiments are directed to a composition that includes acrosslinked polymer. The crosslinked polymer includes a polysiloxane(e.g., polydimethylsiloxane) and at least two dimerizablehydroxycinnamic acid crosslinkers. The crosslinkers can have structuressuch as

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane. In some embodiments, R,R′, and R″ can each be a hydrogen atom, an acyl group, an acryloylgroup, an allyl group, or a propanethiol group. The dimerizablehydroxycinnamic acid crosslinkers can be hydroxycinnamic acids, alcoholsderived from hydroxycinnamic acids, and functionalized hydroxycinnamicacid derivatives. The polysiloxane can be crosslinked with a dimerizedcrosslinker having the following structure:

Additional embodiments are directed to a method of forming a crosslinkedpolymer. The method includes obtaining a polysiloxane (e.g.,polydimethylsiloxane) and at least two dimerizable hydroxycinnamic acidcrosslinkers, reacting the polysiloxane and the crosslinkers to form acrosslinked polysiloxane, and dimerizing the at least twohydroxycinnamic acid crosslinkers. The dimerizing can include reactingthe dimerizable hydroxycinnamic acid crosslinkers with light atwavelengths at or below about 260 nm. The dimerizable hydroxycinnamicacid crosslinkers can be derived from a hydroxycinnamic acid such ascoumaric acid, sinapic acid, ferulic acid, caffeic acid, fertaric acid,coutaric acid, caftaric acid, cichoric acid, chlorogenic acid, anddiferulic acid. The hydroxycinnamic acid can be obtained from a biobasedsource. The polysiloxane can have at least two repeat units with afunctional group (e.g., a vinyl group, a methoxy group, a hydrogen atom,a hydroxyl group, or an alkyl group) that can participate in acrosslinking reaction (e.g., a thiol-ene reaction, a condensationreaction, or an addition reaction) with the at least two dimerizablehydroxycinnamic acid crosslinkers.

Further embodiments are directed to an article of manufacture thatincludes a crosslinked polymer. The crosslinked polymer includes apolysiloxane and at least two dimerizable hydroxycinnamic acidcrosslinkers. The crosslinkers can have structures such as

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane. In some embodiments, R,R′, and R″ can each be a hydrogen atom, an acyl group, an acryloylgroup, an allyl group, or a propanethiol group. The polysiloxane can becrosslinked with the at least two hydroxycinnamic acid crosslinkers.Dimerization of the crosslinked crosslinkers can form a dimerizedcrosslinker having the following structure:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a process of forming a crosslinkedpolysiloxane, according to some embodiments of the present disclosure.

FIG. 2 is a chemical reaction diagram illustrating processes of forminghydroxycinnamic acid crosslinkers, according to some embodiments of thepresent disclosure.

FIG. 3 is a chemical reaction diagram illustrating a process ofcrosslinking PDMS via a reaction with a hydroxycinnamic acidcrosslinker, according to some embodiments of the present disclosure.

FIG. 4 is a chemical reaction diagram illustrating a process ofcrosslinking PDMS with a dimerizable crosslinker, according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

Polysiloxanes, such as polydimethylsiloxane (PDMS), are widely usedtypes of silicon-based polymers. Examples of PDMS applications caninclude medical devices, contact lenses, microfluidic andmicroelectromechanical systems, soft lithography processes, shampoos,caulking, lubricants, foods and cooking oils, etc. PDMS is viscoelasticmaterial, and has tunable properties such as mechanical properties(e.g., viscosity, shear modulus, elasticity, etc.) and chemicalproperties (e.g., surface chemistry, functionalization, etc.) dependingon factors such as the number (n) of [SiO(CH₃)₂]_(n) repeat units,presence and number of functionalized repeat units, and degree ofcrosslinking throughout the polymer matrix. The type of crosslinking isdetermined, at least in part, by the molecules used as crosslinkers. Byvarying the functional groups and structures of organic crosslinkers,PDMS materials having numerous properties can be produced.

However, the organic crosslinkers used with PDMS are typically derivedfrom petrochemical sources. Due to the rising costs of fossil fuels andincreasing environmental regulatory controls, it can be advantageous toreplace traditional petrochemical crosslinkers with crosslinkers fromsustainable sources, such as biomass. Advances in biotechnology haveprovided numerous strategies for efficiently and inexpensively producingbio-based compounds on an industrial scale. Examples of these strategiescan be found in fermentation technologies, membrane technologies, andgenetic engineering. Biotechnological strategies can include plant-basedand microorganism-based approaches. Plant-based approaches can involveobtaining a material directly from a plant, or growing plant tissues orcells that can produce bio-based compounds from various substrates usingtheir own biosynthetic pathways. Microorganism-based approaches involveusing native or genetically modified fungi, yeast, or bacteria toproduce a desired compound from a structurally similar substrate.

Disclosed herein are techniques for forming crosslinkers derived fromhydroxycinnamic acids. The hydroxycinnamic acids are functionalized withvarious moieties in order to produce the crosslinkers. In someembodiments, the functionalization reactions employ additional biobasedcompounds as sources of the added moieties. The crosslinkers can bereacted with hydroxy- or methoxy-functionalized PDMS to form a varietyof crosslinked PDMS polymers. In some embodiments, the disclosedcrosslinkers can also be used with polymers other than PDMS (e.g., otherhydroxy- or methoxy-functionalized polymers). Hydroxycinnamic acids andhydroxycinnamates are hydroxy derivatives of cinnamic acid((2E)-3-phenylprop-2-enoic acid), which is an intermediate in a numberof biosynthetic processes. Cinnamic acid can also be obtained fromnatural products such as oil of cinnamon, balsams (e.g., storax), aloe,and shea butter. Cinnamic acid has the following structure:

For illustrative purposes, the phenyl carbon atom positions are numbered1-6. Most hydroxycinnamic acids have a hydroxyl group bound to carbon atphenyl position 3 (e.g., coumaric acid, sinapinic acid, and ferulicacid) or positions 3 and 4 (e.g., caffeic acid, fertaric acid, coutaricacid, caftaric acid, cichoric acid, and chlorogenic acid). Additionally,some hydroxycinnamic acids (e.g., caftaric acid, coutaric acid, fertaricacid, and chlorogenic acid, diferulic acids, α-cyano-4-hydroxycinnamicacid, etc.) can have hydroxyl groups and/or other functionalities boundto the phenyl ring and/or substituents on the ring. An example structureof a hydroxycinnamic acid, caffeic acid, is illustrated in FIG. 2 .Herein, “hydroxycinnamic acid” refers to any hydroxycinnamic acid unlessstated otherwise.

FIG. 1 is a flow diagram illustrating a process 100 of forming acrosslinked polysiloxane, according to some embodiments of the presentdisclosure. A polysiloxane is obtained. This is illustrated at step 110.In some embodiments, the polysiloxane is obtained as monomers forforming the polysiloxane (e.g., in the presence of crosslinkers).However, the polymerized polysiloxane can be obtained at step 110. Insubsequent steps, the polymer can be crosslinked (e.g., during blending,curing, or other processing). This is discussed in greater detail withrespect to FIG. 3 . In some embodiments, the polysiloxane is PDMS.However, other polysiloxanes, such as polymethylhydrosiloxane (PMHS) orpolymethylmethoxysiloxane (PMS), can be used as well

At least one hydroxycinnamic acid crosslinker is obtained. This isillustrated at step 120. Hydroxycinnamic acid crosslinkers can behydroxycinnamic acids, alcohols obtained by reducing hydroxycinnamicacids, and/or functionalized compounds derived from hydroxycinnamicacids. The hydroxycinnamic acids can be isolated from a variety ofbiological sources. For example, caffeic acid can be obtained fromhawthorn, burdock, artichokes, pears, basil, thyme, apples, and oregano.Sinapinic acid, caftaric acid, coutaric acid, and fertaric acid can beobtained from wine and grapes. Ferulic acid can be obtained from flax,bran, legumes, oats, rice, artichokes, oranges, pineapples, apples, andpeanuts. Chlorogenic acid can be obtained from echinacea, strawberries,pineapples, coffee, sunflowers, and blueberries. However,hydroxycinnamic acids can be obtained from any appropriate source (e.g.,vinegar and additional plant sources) or synthetic technique (e.g.,hydroxylation of cinnamic acid or saponification of hydroxycinnamates).

Functionalization of the hydroxycinnamic acids can be carried out viareactions involving the hydroxyl groups and/or carboxylic acid groups onthe hydroxycinnamic acid. The functional groups are selected based ontheir ability to bind at selected reactive sites on a polysiloxanechain. Binding to a polysiloxane chain is discussed in greater detailwith respect to FIG. 3 . Examples of functional groups that can beattached at phenyl positions 1, 3, 4 of the hydroxycinnamic acid caninclude acyls, thiols (e.g., via ether or ester linkages), acetates,vinyl acetates, vinyl ethers, etc. In some embodiments, the carboxylicacid group(s) on the hydroxycinnamic acid are reduced to hydroxylsbefore attachment of new functional groups to form the crosslinkers.Sources of the functional groups can include a variety of organiccompounds (e.g., allyl alcohol, acrylic acid, acetic acid,3-mercaptopropionic acid, etc.). In some embodiments, the organiccompounds are derived from biomass or other bio-based sources. This isdiscussed in greater detail with respect to FIG. 2 .

A crosslinked polysiloxane is formed. This is illustrated at step 130.The polysiloxane can optionally contain additional components such asbound or unbound additives (e.g., reactive and/or additive flameretardants, fillers, chain extenders, other crosslinkers, pigments,dyes, blowing agents, nano- and/or microcapsules, nano- and/ormicroparticles, etc.). A variety of techniques can be used to form thecrosslinked polysiloxane. For example, vinyl functionalized crosslinkerscan be applied to a polysiloxane via addition and peroxide curereactions, which use platinum and peroxide catalysts, respectively.Hydroxycinnamic acid crosslinkers having at least two hydroxyl groups,as well as acetate functionalized hydroxycinnamic acid crosslinkers, canbe applied to the polysiloxane via condensation cure reactions.Thiol-functionalized crosslinkers can be applied to the polysiloxane inthiol-ene cure reactions.

In some embodiments, the curing reactions are carried out duringprocessing of the polysiloxane, resulting in the crosslinkedpolysiloxane. In other embodiments, the crosslinkers can be mixed withthe polysiloxane to form a partially crosslinked or uncrosslinkedpolysiloxane. The curing can then be carried out in response to asubsequent action. For example, polysiloxane-based caulking and coatingscan be cured once applied to a surface. In some embodiments, curingrequires or is aided by application of heat. Curing reactions arediscussed in greater detail with respect to FIG. 3 . In someembodiments, the crosslinking at step 130 can include dimerization ofthe hydroxycinnamic acid crosslinkers. This is discussed in greaterdetail with respect to FIG. 4 .

FIG. 2 is a chemical reaction diagram 200 illustrating processes 201,202, and 203 of forming hydroxycinnamic acid crosslinkers, according tosome embodiments of the present disclosure. For simplicity, caffeic acid210 is the only hydroxycinnamic acid illustrated in FIG. 2 . However,processes 201, 202, and 203 can be carried out with otherhydroxycinnamic acids, resulting in analogous derivatives. Examples ofhydroxycinnamic acids that can be used are discussed in greater detailwith respect to FIG. 1 .

In process 201, caffeic acid 210 is reacted to form a caffeicacid-derived crosslinker 220 (“caffeic acid crosslinker 220”. Theillustrated caffeic acid crosslinker 220 has substituents at phenylpositions 1, 3, and 4, which include, respectively, functional groupsreferred to as R, R′, and R″. Each functional group can bind to anappropriate functional group on PDMS (see FIG. 3 ). Therefore, eachmolecule of the caffeic acid crosslinker 220 may bind to PDMS at threepositions on the PDMS chain. In other embodiments, a substituent on thecaffeic acid crosslinker 220 does not have a functional group that canform a bond with a functional group on the PDMS chain. In theseinstances, the caffeic acid crosslinker 220 can instead bind at twopositions on the PDMS chain. Further, caffeic acid crosslinkers 220having at least one substituent with more than one functional group canbe formed. These caffeic acid crosslinkers 220 may bind to PDMS at morethan three positions. Similar crosslinkers formed from hydroxycinnamicacids with additional functional groups (e.g., caftaric acid, coutaricacid, fertaric acid, ferulic acid, chlorogenic acid, cichoric acid,etc.) may also form more than two bonds.

In some embodiments, process 201 is a reaction between caffeic acid 210and acetic acid or an acetate (e.g., methyl acetate). The acetic acid oracetate can be obtained from any source. In some embodiments, aceticacid and/or methyl acetate are derived from biomass. For example, aceticacid can be produced through bacterial fermentation. Methyl acetate canbe produced by esterification of acetic acid in the presence of a strongacid (e.g., sulfuric acid). In other embodiments, the acetic acid and/oracetate are produced via conventional synthetic techniques. Further,acetic acid and acetates may be obtained from commercial sources.

When process 201 is a reaction between caffeic acid and acetic acid ormethyl acetate, the caffeic acid crosslinker 220 can have a structurewhere R is a hydrogen atom and R′ and R″ are acyl radicals. That is,this crosslinker (“crosslinker A”) can include the carboxylic acid groupat phenyl position 1 and acetate groups at phenyl positions 3 and 4. Thecarboxylic acid group at position 1 can be reacted further to form asubstituent with a terminal acetate group. For example, crosslinker Acan be reacted with potassium carbonate (K₂CO₃) and 3-bromopropylacetate in a mixture of acetone and dimethylsulfoxide (DMSO) to form anacetate-functionalized crosslinker (“crosslinker B”) having thefollowing structure:

Process 201 can also be a reaction between caffeic acid 210 and allylbromide (3-bromo-1-propene), resulting in a crosslinker 220 having allylfunctional groups. For example, caffeic acid 210 can be reacted withallyl bromide and K₂CO₃ in an acetone/DMSO mixture, resulting in acrosslinker (“crosslinker C”), where R, R′, and R″ are allyl groups(—CH₂CH═CH₂), which contain vinyl groups (—CH₂═CH₂). In someembodiments, the allyl bromide is derived from biobased allyl alcohol(prop-2-en-1-ol). For example, allyl alcohol can be produced bydeoxygenating glycerol obtained as a biodiesel byproduct. Thedeoxygenation reaction can include treatment of glycerol with formicacid at about 230° C.-240° C. However, allyl alcohol can also beobtained via other techniques, such as hydrolysis of allyl chloride orallyl iodide. Conversion of allyl alcohol to allyl bromide can becarried out using techniques known to persons of ordinary skill in theart (e.g., via reaction of allyl alcohol and hydrobromic acid in thepresence of sulfuric acid).

Process 201 can be a reaction between caffeic acid 210 and acrylic acid.The acrylic acid can be obtained from any source. In some embodiments,the acrylic acid is derived from biomass. For example, biobased glucosecan be fermented to form lactic acid. The lactic acid can then bereacted in an acid-catalyzed reaction to form acrylic acid. Thisacid-catalyzed reaction can take place in an azeotropic solvent, such asacetic acid, at a temperature and pressure of approximately 70° C. andapproximately 2×10⁴ Pa, respectively.

When process 201 is the reaction between caffeic acid 210 and acrylicacid, the caffeic acid crosslinker 220 can have a structure in which Ris a hydrogen atom and R′ and R″ are acryloyl (prop-2-enoyl) moieties.That is, this crosslinker (“crosslinker D”) can include the carboxylicacid substituent at phenyl position 1 and acrylates at positions 3 and4. The carboxylic acid group can be reacted further to form anester-linked substituent where R is an allyl group. For example,crosslinker D can be reacted with K₂CO₃ and allyl bromide in anacetone/DMSO mixture to form a vinyl-functionalized crosslinker(“crosslinker E”) having the following structure:

In further embodiments, caffeic acid 210 can be reacted with3-bromopropyl thiolacetate (CH₃C(═O)S(CH₂)₃Br) and K₂CO₃ in a mixture ofacetone and DMSO, followed by refluxing in a hydrochloric (HCl)acid/ethanol (EtOH) mixture. This reaction can produce a caffeic acidcrosslinker 220 having a structure where R, R′, and R″ are eachpropanethiol groups (—CH₃(CH₂)₂SH). That is, this crosslinker(“crosslinker F”) has an ester-linked thiol substituent at phenylposition 1 and ether-linked thiol substituents at phenyl positions 3 and4.

A caffeic acid crosslinker 220 with ester-linked thiol substituents atphenyl positions 1, 3, and 4 can be produced as well. For example,caffeic acid 210 can be reacted with 3-mercaptopropionic acid to producea caffeic acid crosslinker 220 having a structure with a carboxylic acidgroup at phenyl position 1 and ester-linked thiol substituents at phenylpositions 3 and 4. This carboxylic acid- and thiol-functionalizedcrosslinker (“crosslinker G”) can be reacted with 3-bromopropylthioacetate and K₂CO₃ in an acetone/DMSO mixture, followed by refluxingin an HCl/EtOH mixture to produce a thiol-functionalized crosslinker(“crosslinker H”) with the following structure:

Referring now to processes 202, the caffeic acid 210 carboxylate groupis reduced to form an alcohol (caffeyl alcohol 230). The reductionreaction can be carried out using any appropriate techniques forreducing a carboxylic acid moiety to form a hydroxyl group. For example,caffeic acid 210 can be reacted with lithium aluminum hydride (LiAlH₄),followed by protonation, to form the hydroxyl group. Additionaltechniques for carboxylic acid reduction are known to those of ordinaryskill in the art. Reduction of other hydroxycinnamic acids results inanalogous alcohols. For example, ferulic acid can be reduced toconiferyl alcohol (4-hydroxy-3-methoxycinnamyl alcohol), and sinapicacid can be reduced to sinapyl alcohol (4-hydroxy-3,5-dimethoxycinnamylalcohol).

In process 203, caffeyl alcohol 230 can be reacted further to produce acaffeyl alcohol crosslinker 240 with functional groups R, R, and R″.Reactions substantially the same as those used to modify the hydroxylgroups at positions 3 and 4 in process 201 can be carried out withcaffeyl alcohol 230 to bind functional groups R, R′, and R″ tosubstituents at phenyl positions 1, 3, and 4, respectively. Therefore,the caffeyl alcohol crosslinker 240 formed in process 203 can haveether-linked moieties (e.g., allyl, acyl, propanethiol, acryloyl, thioletc.) for each of R, R′, and R″. In one example, caffeyl alcohol 230 canbe reacted with methyl acetate to produce a caffeyl alcohol crosslinker240 where R, R′, and R are acyl groups. Additional reactions that can becarried out to form hydroxycinnamic acid crosslinkers derived fromcaffeyl alcohol 230 are discussed in greater detail above with respectto process 201.

FIG. 3 is a chemical reaction diagram illustrating a process 300 ofcrosslinking PDMS 310 via a reaction with a hydroxycinnamic acidcrosslinker (e.g., 210, 220, 230, and/or 240 of FIG. 2 ), according tosome embodiments of the present disclosure. The illustrated crosslinkeris a caffeyl alcohol crosslinker 240. However, any appropriatehydroxycinnamic acid crosslinker can be used. For example, caffeic acidcrosslinkers 220 formed in process 201, alcohols (e.g., caffeyl alcohol230), and hydroxycinnamic acids (e.g., caffeic acid 210) can be added toPDMS 310 in process 300 as well. In these instances, the hydroxycinnamicacid crosslinkers will have ester linkages to the PDMS 310 at phenylposition 1. An example of this is illustrated in FIG. 4 .

PDMS 310 is illustrated with a number (n) of dimethyl repeat units,where n can be an integer between 1 and 1,000,000 (e.g., about 1-25,000,about 500-25,000, about 1,000-25,000, about 2,000-25,000, about5,000-25,000, about 25,000-50,000, about 50,000-500,000, etc.), and onefunctionalized repeat unit, where X can represent a functional groupother than methyl. However, this is for simplicity, and it should beunderstood that crosslinking occurs when a crosslinker binds to morethan one functionalized repeat unit on the PDMS 310. PDMS 310 propertiescan be tuned by varying the number and positions of functionalizedrepeat units, as will be understood by persons of ordinary skill in theart. In some embodiments, at least one of the functionalized repeatunits can have a different X functional group that another repeat unit.For example, a molecule of PDMS 310 can have at least one repeat unitwhere X is a methoxy group and at least one repeat unit where X is ahydroxyl group.

In process 300, hydroxycinnamic acid crosslinkers (e.g., 210, 220, 230,240, etc.) are added to PDMS 310 to form the crosslinked PDMS 320. Thecrosslinked PDMS 320 includes a bond to a crosslinker at the position ofthe functionalized repeat unit on PDMS 310. The additional crosslinkingbonds are illustrated at additional functionalized repeat unitsrepresented by wavy lines in FIG. 3 . Various techniques can be used toform the crosslinked PDMS 320 in process 300.

For example, vinyl-functionalized crosslinkers can be used to crosslinkunsubstituted PDMS 310 and PDMS 310 where X is another alkyl group(e.g., ethyl, propyl, isopropyl, etc.) via addition/peroxide curereactions. Examples of vinyl-functionalized crosslinkers that canparticipate in the addition/peroxide cure reactions can includecrosslinkers C, D, and E, analogous vinyl-functionalized crosslinkersderived from hydroxycinnamic acids other than caffeic acid 210, andanalogous crosslinkers derived from caffeyl alcohol 230 or other reducedhydroxycinnamic acid derivatives (e.g., coniferyl alcohol, sinapylalcohol, etc.). The addition/peroxide cure reactions in process 300 canbe carried out with platinum catalysts (e.g., Karstedt's catalyst),peroxide initiators (e.g., di-tert-butyl peroxide, benzoyl peroxide,methyl ethyl ketone peroxide (MEKP), peroxydisulfate salts, etc.), etc.

In other examples, PDMS 310 can be crosslinked in a condensation curereaction. In some embodiments, these condensation reactions can becatalyzed by dibutyltin dilaurate (DBTDL). Acetate-functionalizedcrosslinkers (e.g., crosslinkers A or B and analogousacetate-functionalized crosslinkers derived from reduced compounds suchas caffeyl alcohol 230, coniferyl alcohol, sinapyl alcohol, etc.) can beused in a condensation cure with PDMS 310 where X is a hydroxyl group.Condensation cures can also be carried out with hydroxyl-functionalizedcrosslinkers and PDMS 310 where X is a hydrogen atom or a methoxy group.Hydroxyl-functionalized crosslinkers that can be used can includecaffeyl alcohol 230, coniferyl alcohol, sinapyl alcohol, and otheralcohols derived from hydroxycinnamic acids. In some embodiments,hydroxycinnamic acids with more than one hydroxyl group, such ascichoric acid, can be used as well.

Thiol-ene cure reactions can also be used in process 300. Thiol-enereactions can be used to crosslink polymers having alkene groups withthiol-functionalized crosslinkers. For example, process 300 can includePDMS 310, where X is a vinyl group, a substituent containing a vinylgroup (e.g., allyl), or another alkene substituent, andthiol-functionalized crosslinkers (e.g., crosslinker F, G, and H). Insome embodiments, the thiol-ene reactions occur via free-radicaladdition. Free-radical addition reactions can be initiated byapplication of light (e.g., ˜250 nm-300 nm), heat (e.g., heated air orinfrared radiation), or radical initiators (e.g., halogens, peroxides,azo compounds, etc.). In other embodiments, the thiol-ene reactionsoccur via Michael addition. Michael addition reactions can be catalyzedby primary or tertiary amine catalysts, phosphine catalysts, etc.

FIG. 4 is a chemical reaction diagram illustrating a process 400 ofcrosslinking PDMS with a dimerizable crosslinker, according to someembodiments of the present disclosure. Examples of reactions that can beused to crosslink PDMS with the dimerizable crosslinker 410 arediscussed in greater detail with respect to FIG. 3 . The crosslinkingbonds of the dimerizable crosslinker 410 are illustrated at bindingsites on functionalized repeat units represented by wavy lines. Theillustrated dimerizable crosslinker 410 has an ester linking group atphenyl position 1, and can be a functionalized hydroxycinnamic acidcrosslinker (e.g., caffeic acid crosslinker 220 or 240). However, thedimerizable crosslinker 410 can also be a hydroxycinnamic acid (e.g.,caffeic acid 210), an alcohol (e.g., caffeyl alcohol 230), or analcohol-derived crosslinker (e.g., a caffeyl alcohol crosslinker 240).Compounds 210, 220, 230, and 240 are illustrated in FIG. 2 . When analcohol or alcohol-derived crosslinker is used, the dimerizablecrosslinker 410 will include an ether linking group at phenyl position1.

In process 400, dimerization of the dimerizable crosslinker 410 isintroduced by application of light (hv) to crosslinked PDMS matrix.Application of ultraviolet (UV), such as light at or below approximately260 nm (e.g., ˜100 nm-260 nm) causes a dimerization reaction between twomolecules of the dimerizable crosslinker 410, resulting in a dimerizedcrosslinker 420. In some embodiments, the dimerization reaction can becarried out at wavelengths of light below approximately 300 nm. Thedimerization reaction can be reversed in the presence of light withwavelengths at or above approximately 300 nm-315 nm (e.g., ˜300-700 nm,˜400-800 nm, etc.).

In addition to the compounds and reactions discussed with regard toFIGS. 2, 3, and 4 , hydroxycinnamic acids and crosslinked polysiloxanescan be formed using numerous additional functional groups and/orpolymerization, processing, and curing techniques. For example,crosslinkers can be formed with R, R′, and/or R″ moieties includingfunctional groups such as epoxides, propylene carbonate, alkyl halides,esters, alkynes, amines, isocyanates, acid chlorides, chloroformates,etc. Additional polysiloxane functional groups and crosslinkingtechniques that can be used with the hydroxycinnamic acid crosslinkerfunctional groups are known to persons of ordinary skill in the art.

R, R′, and/or R″ can also include moieties such as substituted orunsubstituted alkyl groups. Herein, “alkyl” refers to C₁-C₁₀₀ radicals,which can be linear, branched, or cyclic. Examples of alkyl groups caninclude methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl,sec-butyl, tert-butyl, pentyl, hexyl, octyl, cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, cyclooctyl. In some embodiments, the alkyls areunsaturated (e.g., alkenes and alkynes).

In some embodiments, R, R′, and/or R″ can also include moieties such ascyclic alkyl groups and substituted analogues of cyclic alkyl group.When cyclic, the alkyl groups can be aromatic or non-aromatic. Herein,the term “aromatic” also refers to pseudoaromatic heterocycles, whichare heterocyclic substituents that have similar properties andstructures (nearly planar) to aromatic heterocyclic ligands, but are notby definition aromatic. Examples of cyclic aromatic alkyls that can beused can include six-carbon aromatic rings (phenyl) and substitutedvariants thereof (e.g. 2-methyl-phenyl, xylyl, tolyl, etc.), C₄-C₆₀aromatic rings, C₄-C₂₀ aromatic rings, etc. The cyclic groups canoptionally include heteroatoms (e.g., nitrogen, oxygen, or sulfur)replacing at least one carbon atom in the cyclic structure.

The compounds described herein can contain one or more chiral centers.Unless otherwise noted, the disclosed structures cover allstereoisomers, conformers, rotamers, isomers, and enantiomers of therepresented compounds. Further, polymers or other materials containingthe disclosed compounds can include racemic forms of the compounds inaddition to individual stereoisomers, as well as mixtures containing anyof these. Substituents on the compounds described herein may participatein additional chemical reactions, transformations, or interactions,which can include synthesis, decomposition, single and/or doublereplacement, oxidation/reduction, acid/base, nucleophilic, electrophilicand radical substitutions, addition/elimination reactions, andpolymerization reactions.

Where isomers of a named alkyl, alkenyl, alkoxy, aryl, or otherfunctional group exist (e.g., n-butyl, iso-butyl, sec-butyl, andtert-butyl), reference to a member of the group without specifying aparticular isomer (e.g., butyl) is intended to include all isomers inthe family (e.g., n-butyl, iso-butyl, sec-butyl, and tert-butyl).Further, unless specified otherwise, reference to one member of thegroup (e.g., n-butyl) includes the remaining isomers in the family(e.g., iso-butyl, sec-butyl, and tert-butyl).

Unless otherwise noted, chemical reactions are performed at ambientconditions or under slight heating with no special atmosphere or headspace, and may be performed using standard organic solvents to managemix properties such as viscosity and flow index. Standard procedures forquenching reactions, solvent removal, and purification are performed.Room temperature is between about 15° C. and 25° C. unless otherwiseindicated. Ranges (e.g., time, concentration, temperature, etc.)indicated herein include both endpoints and all numbers between theendpoints. Unless specified otherwise, the use of “about,”“approximately,” or a tilde (˜) in connection with a range applies toboth ends of the range (e.g., “approximately 1 g-5 g” should beinterpreted as “approximately 1 g-approximately 5 g”). Unless otherwiseindicated, modifying terms such as “about,” “approximately,” and “˜”indicate +/−10% of a recited value, range of values, or endpoints of oneor more ranges of values.

The processes discussed herein, and their accompanying drawings, are notto be construed as limiting. One skilled in the art would recognize thata variety of techniques may be used that vary in conditions, components,methods, etc., which ultimately generate hydroxycinnamic acidcrosslinkers and crosslinked polysiloxanes. In addition, the conditionscan optionally be changed over the course of a process. Further, in someembodiments processes can be added, omitted, or carried out in alternateorders, while still remaining within the scope of the disclosure, aswill be understood by a person of ordinary skill in the art. It shouldalso be noted that processes can be carried out by a single entity, orby multiple entities. For example, a first entity may form thehydroxycinnamic acid crosslinkers, and a second entity may carry out thecrosslinking of the polysiloxane.

What is claimed is:
 1. A composition comprising a crosslinked polymer,the crosslinked polymer derived from materials comprising: apolysiloxane, and dimerizable hydroxycinnamic acid-derived crosslinkers.2. The composition of claim 1, wherein the dimerizable hydroxycinnamicacid-derived crosslinkers have the following structure:

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane and wherein R, R′, and R″are each selected from the group consisting of a hydrogen atom, an acylgroup, an acryloyl group, an ally group, and a propanethiol group. 3.The composition of claim 1, wherein the dimerizable hydroxycinnamicacid-derived crosslinkers are selected from the group consisting ofhydroxycinnamic acids, alcohols derived from reacting hydroxycinnamicacids with a reducing agent, and functionalized hydroxycinnamic acidderivatives.
 4. The composition of claim 1, wherein, upon dimerizationof the dimerizable hydroxycinnamic acid-derived crosslinkers, thecrosslinked polymer has the following structure:


5. The composition of claim 1, wherein the dimerizable hydroxycinnamicacid-derived crosslinkers have the following structure:

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane and wherein R, R′, and R″are each selected from the group consisting of a hydrogen atom, an acylgroup, an allyl group, an acryloyl group, and a propanethiol group.
 6. Amethod of forming a crosslinked polymer, comprising: obtaining apolysiloxane; obtaining dimerizable hydroxycinnamic acid-derivedcrosslinkers; reacting the polysiloxane and the dimerizablehydroxycinnamic acid-derived crosslinkers to form a crosslinkedpolysiloxane; and dimerizing the dimerizable hydroxycinnamicacid-derived crosslinkers.
 7. The method of claim 6, wherein thedimerizing includes reacting the dimerizable hydroxycinnamicacid-derived crosslinkers with light at wavelengths at or belowapproximately 260 nm.
 8. The method of claim 6, wherein the dimerizablehydroxycinnamic acid-derived crosslinkers are selected from the groupconsisting of coumaric acid, sinapic acid, ferulic acid, caffeic acid,fertaric acid, coutaric acid, caftaric acid, cichoric add, chlorogenicacid, diferulic acid, and derivatives thereof.
 9. The method of claim 6,wherein the dimerizable hydroxycinnamic acid-derived crosslinkers areobtained from a biobased source.
 10. The method of claim 6, wherein thepolysiloxane has at least two repeat units with functional groups thatcan participate in a crosslinking reaction with the dimerizablehydroxycinnamic acid-derived crosslinkers.
 11. The method of claim 10,wherein the functional groups are selected from the group consisting ofa vinyl group, a methoxy group, a hydrogen atom, a hydroxyl group, andan alkyl group.
 12. The method of claim 6, wherein the reacting thepolysiloxane and the dimerizable hydroxycinnamic acid-derivedcrosslinkers includes a reaction selected from the group consisting of athiol-ene reaction, a condensation reaction, and an addition reaction.13. An article of manufacture comprising a crosslinked polymer, thecrosslinked polymer derived from materials comprising: a polysiloxane,and dimerizable hydroxycinnamic acid-derived crosslinkers.
 14. Thearticle of manufacture of claim 13, wherein the dimerizablehydroxycinnamic acid-derived crosslinkers have the following structure:

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane, and wherein R, R′, and R″are each selected from the group consisting of a hydrogen atom, an acylgroup, an allyl group, an acryloyl group, and a propanethiol group. 15.The article of manufacture of claim 13, wherein the dimerizablehydroxycinnamic acid-derived crosslinkers have the following structure:

where —OR, —OR′, and —OR″ are substituents that can participate incrosslinking reactions with the polysiloxane, and wherein R, R′, and R″are each selected from the group consisting of a hydrogen atom, an acylgroup, an allyl group, an acryloyl group, and a propanethiol group. 16.The article of manufacture of claim 13, wherein, upon dimerization ofthe dimerizable hydroxycinnamic acid-derived crosslinkers, thecrosslinked polymer has the following structure:

wherein the wavy lines represent binding sites on the polysiloxane.